Method of manufacturing ferroelectric ceramic

ABSTRACT

NOVEL PB(NB2/3ZN1/3)O3-PBTIO3-PBZRO3 COMPOSITIONS WITH SUPERIOR ELECTROMECHANICAL COUPLING COEFFICIENTS ARE PROVIDED BY OPTIMIZATION OF THE PRE-FIRING TEMPERATURE. THE OPTIMUM PRE-FIRING TEMPERATURE IS DETERMINED AS A FUNCTION OF THE STOICHIOMETRY ACCORDING TO THE FOLLOWING FORMULA   TP=850-3(X-0.087(2Z+X-100))$10   WHEREIN   TP IS THE PRE-FIRING TEMPERATURE X IS THE MOL PERCENT OF PB(NB2/3 ZN1/3)O5 Z IS THE MOL PERCENT OF PBZRO3.

Sept. 5, 1972 HIDEO WATANABE ETAI- 3,589,414

METHOD OF MANUFACTURING FERROELECTRIC CERAMIC Filed April 27, 1970 2 Sheets-Sheet 1 Pb (Nb 211 /90 50 50 MW 40 2b 60 7O 24 WA [WWWWWNAAA INVENTORS H LDEO WATA NABE YOSH IYU KI. YAGI ATTORNEYS Sept 5, 1972 Filed April 27, 1970 Pb(Nb2 HIDEO WATANABE EIAL METHOD OF MANUFACTURING l rmnoleluccnx1c CERAMIC 2 Sheets-Sheet I INVENTORS HIDEO WATANABE g YOSH wum YAGI pama ATTORNEYJ United States Patent US. Cl. 252-623 2 Claims ABSTRACT OF THE DISCLGSURE Novel Pb (Nb Zn O -PbTiO -PbZrO compositions with superior electromechanical coupling coefficients are provided by optimization of the pre-firing temperature. The optimum pre-firing temperature is determined as a function of the stoichiometry according to the following formula T =8503 [x-0.087(2z+x100) i 10 wherein T is the pre-firing temperature x is the mol percent of Pb(Nb -Zn )O Z is the mol percent of PbZrO This is a continuation-in-part application of copending application Ser. No. 696,768, filed Jan. 10, 1968, now abandoned.

This invention relates to a novel method of manufacturing piezoelectric ceramic.

An object of this invention is to provide a piezoelectric ceramic, which possesses an electromechanical coupling coefiicient higher than those of the known piezoelectric ceramics, satisfactory large values of dielectric constant and piezoelectric modulus and sufiiciently high Curie temperature which is important for a polycrystalline piezoelectric material.

Another object of this invention is to provide a piezoelectric ceramic, which is easily sintered and possesses extremely high-density as a ceramic containing lead, and therefore, exhibits desirable insulation reisistance.

Still another object of this invention is to provide a novel method of manufacturing a piezoelectric ceramic having useful ferroelectric properties as above-described,

i.e., according to this invention, it is possible to obtain a piezoelectric ceramic exhibiting properties that are constant in quality of products manufactured by mass-pro-' duction in the industry and that are desirable for use in various electrocommunication devices.

piezoelectric materials. Several materials such as barium titanate BaTiO potassium niobate KNbO' and so forth were discovered to be useful in production of piezoelectric ceramics. Among these materials, solid solutions of lead titanate and lead zirconate Pb(TiZr)O ,.both of which have a perovskite type structure, show excellent piezoelectric properties. The solid solutions of this system, which are near the morphotropic boundaries, exhibit peculiar properties, and can be employed in practical use. In this connection, some improvements on this lead titanozirconate ceramic Pb(TiZr) O have been achieved by various modifications, such as addition of a small amount of additive, which changes the characteristic values, sub- 3,689,414 Patented Sept. 5, 1972 stitution for plumbons ions Pb++ of other divalent ions such as Sr++, Ca++, etc., which improves the sintering properties, and so forth. The compositions in the ternary system consisting of the above mentioned binary system and lead stannate PbSnO which have perovskite-type structures represents a further modification. The compositions on the morphotropic boundaries show good piezoelectric properties, but their electromechanical coupling coefiicient is not sutficiently high.

Furthermore, such solid solutions as above have disadvantages in that it is rather difficult to obtain a ceramic having high density. It has been desired by the prior art that the piezoelectric ceramics have consistent characteristics and quality. Up to the present time, since characteristics of a ceramic containing lead are generally affected even by slight variation of manufacturing conditions, such piezoelectric ceramics could not be produced by massproduction.

In recent years, the demand for piezoelectric ceramics has greatly increased. Electromechanical transducers embodying such piezoelectric ceramics are employed in ultrasonic generators, audio devices, transducers of electromechanical filters, ceramic filters and so forth. Presently, the characteristics required for oscillators, which are used in non-resonance systems such as pick-up microphones in the audio range, are such that both their electromechanical coupling coefficient K and piezoelectric constant d are high whereas they have a relatively low mechanical Q-value.

A feature of the present invention is directed to a novel method of manufacturing a electric ceramic composed of a ternary solid solution comprising ferroelectric lead niobo-zincate Pb(Nb Zn )O with a perovskite-type structure, lead titanate PbTiO and lead zirconate PbZrO This solid solution has sufficiently high values of electromechanical coupling coefficient, sufficiently high piezoelectric constant, high Curie temperature which is very important to polycrystalline piezoelectric materials.

The piezoelectric ceramic composition according to this invention is produced in the manner hereinafter described. The starting materials are powdered PbO, TiO ZrO Nb O ZnO or the compound obtained by heating and oxidizing the corresponding metals thereof. Particularly, Nb O is preferably used in the state of powder of a particle diameter less than l/J.. The Nb O is produced by heating Nb at a temperature of less than 900 .C., and preferably 600 C.

The raw materials are combined so that the composition is in the range of less than mol percent of lead titanate PbTiO less than 80 mol percent of lead zirconate PbZrO and 0 to mol percent of lead niobozincate Pb(Nb Zn )O and then said powders are mixed with Water and are pre-fired at the pre-firing temperature defined or determined by the following formula based on the composition of the material to be pre-fired Tp=8503 [x0.087(2z+x 100) i 10 wherein:

T is the pre-firing temperature C.) x is the mol percent of Pb(Nb Zn )O and z is the mole percent of PbZrO The material is then crushed and milled into a fine powder. After a binder is added, said fine powder is pressed into a desired shape. The product thus obtained is fired again at a desired temperature according to the composition of product for 2 hours. This secondary firing is performed in a closed furnace. After setting a pair of electrodes on both sides of the fired disk-shaped product, the product is polarized by applying DC. voltage of 3000-450O v./mm. in a silicone oil bath at 80-l40 C.

3 Appended drawings and exemplary but not limitative embodiments are described in the following:

FIG. 1 and FIG. 2 are three component diagrams showing the relationship between the contents of Pb(Nb Zn )O PbTiO and PbZrOg which are the three components in this invention.

EXAMPLE I Pb(Nb2/3ZH1/3)O PbTio 42 PbZrO 48 These materials were wet-mixed in a ball mill for 24 hours. In this case, powder of Nb O of a particle diameter less than l,u (which was obtained by heating at the temperature less than 900 C.) was used. The mix was then prefired at 820 C. in air for 2 hours. The product was ground into powder of a particle diameter less than 1;. Then, an appropriate binder being added, the powder mixture thus obtained was pressed into a disk of 16 mm. in diameter and 1 mm. in thickness, and fired again at 1230 C. for 2 hours. This secondary firing was performed in a closed furnace to prevent evaporation of PbO at the high temperature during the secondary firing. The sintered diskshaped product was used as a piezoelectric body, after having been polarized by a conventional method, for instance by applying 4000 v./mm. static electric field between a pair of electrodes set on both sides of the disk in a silicone oil bath at 120 C.

The characteristic values of the piezoelectric ceramics thus obtained are:

Density- 7.85 g./cm.

Radial electromechanical coupling coefiicient-Kr: 71.5% Mechanical Q value-Qm: 72

Specific dielectric constant-e: 1400 EXAMPLE II Raw materials were the same in Example I. PbO, TiO ZrO Nb and ZnO were provided so that the final composition was:

M01 percent Pb(Nb2/3ZH1/3)O3 30 PbTto 34.5 PbZtO 35.5

These materials were wet-mixed in a ball mill for 24 hours as indicated in Example I, and then pre-fired at 760 C. in air for 2 hours. The product was gound into powder of a particle diameter less than 1 1.. Then, an appropriate binder being added, the powder mixture thus obtained was pressed into a disk of 16 mm. in diameter and 1 mm. in thickness, and fired again at 1220 C. for 2 hours. This secondary firing was performed in a closed furnace to prevent evaporation of PhD at the high temperature during the secondary firing. The sintered diskshaped product was used as a piezoelectric body, after having been polarized by a conventional method, for instance, by applying 4500 v./mm. static electric field between a pair of electrodes set on both sides of the disk in a silicone oil bath at 140 C.

The characteristic value of the piezoelectric ceramics thus obtained are:

Densityp: 7.94 g./cm.

Radical electrometchanical coupling coefficient-Kr: 75.6 Mechanical Q value-Qm: 67

Specific dielectric constante: 1870 EXAMPLE III Raw materials were the same in Example I. PbO, TiO

Pb(Nb Zn 3)O 50 PbTiO 26 PbZrO 24 These materials were wet-mixed in a ball mill for 24 hours as indicated in Example I, and then pre-fired at 700 C. in air for 2 hours. The product was ground into powder of a particle diameter less than 1,11. Then 'an appropriate binder being added, the powder mixture thus obtained was pressed into a disk of 16 mm. in diameter and 1 mm. in thickness, and fired again at 1100" C. for 2 hours. This secondary firing was performed in a closed furnace to prevent evaporation of PhD at the high temperature during the secondary firing. The sintered diskshaped product was used as a piezoelectric body, after having been polarized by a conventional method, for instance by applying 4000 v./mm. static electric field between a pair of electrodes set on both sides of the disk in a silicone oil bath at 120 C.

The characteristic values of the piezoelectric ceramics thus obtained are:

Density-p: 8.02 g./cm.

Radial electrochemical coupling coefficient-Kr: 76.1% Mechanical Q value-Qm: 60

Specific dielectric constante: 2010 Table 1 shows the characteristic values of ceramic bodies according to this invention in various compositions obtained by essentially the same process as the abovementioned examples.

TABLE 1 [J:=Pb(Nba/3. Zm 3)O y=PbTiO z=PbZrO; composition] Component (mol percent) p 0111. K! 40 Specimen No. x y z (gJcmfi) (percent) Qm e 1 42 57 7. 51 45. 1 150 545 1 48 51 7. 48. 2 120 950 1 52 47 7. 52 45. 0 145 550 10 10 80 7. 75 22. 6 350 350 10 25 7. 36. 2 140 580 10 33 57 7. 75 47. 0 135 605 45 10 41 49 7. 76 66. 0 79 960 10 42 48 7. 85 71. 5 72 1, 400 10 43 47 7. 78 70. 7 72 1, 590 10 44 46 7. 79 64. 1 79 580 10 52 38 7. 78 47. 5 130 860 10 65 25 7. 78 30. 6 350 360 10 80 10 7. 77 26. 4 370 310 50 14 10 1 80 7. 68 16. 2 275 360 19 80 1 7. 66 15. 9 370 345 19 25 55 7. 79 43. 5 140 580 20 36 44 7. 66. 0 85 860 20 37 43 7. 87 70. 7 78 1, 030 20 38 42 7. 87 73. 2 70 1, 490 20 39 41 7. 86 71. 5 272 1, 790 20 65 15 7. 83 26. 7 270 375 55 so 32. 5 37. 5 7. 94 69. s 84 955 20 33. 5 36. 5 7. 95 76. 1 81 1, 340 30 34. 5 35. 5 7. 94 77. 1 67 1, 870 40 1 59 7. 89 27. 0 255 395 40 15 45 7. 89 38. 4 175 630 40 30. 5 29. 5 7. 99 74. 2 67 1, 710 40 31. 5 28. 5 8. 01 76. 1 58 1, 950 60 29 40 32. 5 27. 5 8. 00 73. 0 66 2, 280 40 55 5 7. 98 32. 5 240 590 50 18 32 8. 01 49. 8 1, 450 50 26 24 8. 02 76. 1 60 2, 010 50 33 17 8. 00 50. 2 1, 380 50 49 1 7. 98 29. 7 240 780 60 1 39 8. 00 30. 0 310 490 60 15 25 8. 08 40. 1 980 6 5 60 25 15 8. 11 68. 9 80 2, 025 38 60 39 1 8. 08 37. 8 320 850 75 20 5 8. 12 52. 0 2, 100 75 24 1 8. 14 47. 5 205 1, 890 87 12 1 8. 11 49. 7 205 875 Nora-Polarization conditions, 80 -140 0., 3,000-4,500 v./m1n.

described as xPb(Nb Zn )O yPbTO zPbZrO where x-i-y+z==100, is given in each column of x, y and z, and the characteristic values, from left to right, correspond to density of the body p (g./cm. radial electromechanical coupling coefficient Kr (percent); mechanical Q value QM; dielectric constant 6, respectively.

As is apparent from the above-mentioned examples and Table 1, the piezoelectric ceramic produced by the method of this invention is composed of a ternary solid solution comprising ferroelectric lead niobo-zincate and the pre-firing temperature is defined by the following formula:

T the pre-firing temperature C.) 1:: mol percent number of Pb(Nb -Zn 0 z: mol percent number of PbZrO therefore, electric properties, particularly, electromechanical coupling coetficient are sufiiciently high, achieving values of as high as 77% by using this novel method. Furthermore, according to the method of this invention, the density of a ceramic reaches so much as 8 g./cm. which is much the same as the theoretical value. This permits the ceramic of high density to be sintered. (For example, from X-ray studies, it is found that a theoretical value of density of this ceramic is nearly 8.08 g./cm. These features shall be understood from the following Table 2, in which there is shown a ratio of the actual density p of the piezoelectric ceramic to the theoretical density p (It is actually impossible to attain a natio of 100%.)

In Table 2, data of specimen No. 1 is obtained from the literature. As apparently from the Table 2, dual composition comprising lead titanate PbTiO and lead zirconate PbZrO has only 90.0% of theoretical density, whereas addition of only 1 mol percent of lead niobozincate Pb(Nb Zn )O greatly improves the ratio to 95%. Addition of 10 mol percent of Pb (Nb -Zn )0 changes the ratio to 97.0%, and the more the amount of this solid solution is contained, the higher the density of the ceramic is realized.

High density of a ceramic is very important in such polycrystalline piezoelectric material because it is possible to obtain a superior insulating resistance against voltage and easy polarization, i.e., conventional ceramics are polarized at a temperature C.- C. and the insulating resistance against voltage would not permit the temperature to be higher than that because if the temperature is too high, it causes dielectric breakdown. This invention avoids such disadvantages and permits polarization at temperatures of more than C. to C., which enables one to select any desirable temperature according to each composition point. Therefore, its electromechanical coupling coefi'icient becomes extremely high. It is possible to approximate the values of electric properties, i.e., electromechanical coupling coefficient and dielectric constant to theoretical values. It is also improved in moisture resistant properties and specific resistance as well as constant electric properties such as frequency characteristics.

FIG. 2 is three component diagram showing the prefiring temperature defined by the above formula defining FIG. 1 is three-component diagram showing the relationship between the contents of Pb(N b Zn )O PbTiO PbZrO which are the three components in this invention and point numbers in a diagram are corresponding to specimen numbers of Table 1.

What we claim is:

1. A method of manufacturing piezoelectric ceramic consisting essentially of the composition expressed by the formula crushing and milling into a fine powder; admixing a binder; pressing said fine powder and binder mixture into a desired shape, firing said shape in a closed furnace at a temperature of about 1100 C. to about 1230 C., and polarizing said shaped product.

2. A process according to claim 1 wherein the Nb O has a particle size of less than 1,.

References Cited UNITED STATES PATENTS 9/ 1968 Ovchi et a1. 25262.9

TOBIAS E. LEVOW, Primary Examiner I. COOPER, Assistant Examiner US. Cl. X.R. 

